When crushing or grinding rock, ore, cement clinker and other hard materials, roller crushers may be used having two generally parallel rolls which rotate in opposite directions and which are separated by a gap. The material to be crushed is fed by gravity or choke-fed into the gap. One type of roller crusher is called high pressure grinding rollers or high pressure roller crushers. This type of roller crusher uses a crushing technique called interparticle crushing. Here, the material to be crushed or pulverised is crushed, not only by the crushing surface of the rolls, but also by particles in the material to be crushed, hence the name interparticle crushing. One example of a high pressure grinding roller is described in EP-516 952.
A problem which arises in roller crushers is that sometimes objects which are not possible to crush by the roller crusher are present in the in-feed stream of material. If an uncrushable object is fed into the gap of the roller crusher, the rollers and the system holding the rollers may be damaged, causing costs associated with down-time and repair. Therefore, roller crushers are sometimes provided with protective systems for protecting the roller crusher from uncrushable objects. In one known system, shown in FIG. 1, a metal detector 111 is provided at the in-feed 104 to the roller crusher 101. When the metal detector 111 detects an uncrushable object, a by-pass chute 118 is opened, such that the in-feed stream is diverted and by-passes the gap 103 between the rollers 102. The by-pass stream is led to the same discharge chute 110 as crushed material leaving the crushing gap 103. A problem with this solution is that during by-pass mode, the discharge chute 110 receives at least twice the amount of material per unit of time as during normal operation, since the flow of the by-pass stream is added to the flow of crushed material. If several by-pass events occur within a short period of time, the discharge chute 110 is overloaded, or else it has to be severely over dimensioned in relation to the normal operational flow of crushed material. Another problem is that the use of a by-pass chute 118 makes the roller crusher 101 space-requiring. Further, since the by-pass chute 118 is to discharge the by-passed material in the discharge chute 110, the discharge chute 110 needs to be placed at a distance from the exit of the gap 103, thereby adding to the space requirement of the roller crusher 101. Thus, a need remains for an improved method of protecting a roller crusher from uncrushable objects, and for a roller crusher having an improved protective system for protection from uncrushable objects.